Variable vanes, namely vanes which are not fixed but pivot about axes such as to vary the angle of the vane airfoil, are sometimes used in gas turbine engine compressors to optimize compressor operability and/or efficiency over the compressor speed range. These variable vanes may include variable inlet guide vanes (IGV) located directly upstream of the first compressor stage, or variable vanes which form part of one or more of the subsequent downstream stages in a multistage compressor (ex: first compressor stage and/or second compressor stage, etc.). Variable vanes enable optimized compressor efficiency and/or operability by providing a close-coupled direction of the gas flow into the immediately downstream compressor rotor, and/or may introduce swirl into the compressor rotor to improve low speed operability of the compressor, and thus the engine, as well as to increase the flow capacity at high speeds.
Such variable vanes extend between the inner and outer shrouds which define the perimeter of the annular gas path into the compressor, and the variable vanes pivot about their respective radially extending axes to modify the angle of the vane airfoils and thus provide a closer incidence match between the air flow entering exiting the vane and the blade angle of the rotor. However, as each of the variable vane airfoils pivots about its radially extending axis, the clearance gap between the base and tip of the vane airfoil and the surrounding inner and outer shrouds, respectively, also varies. This can lead to greater vane tip losses which may negatively affect the aerodynamic performance of the vanes and thus the compressor. Improvements in variable compressor vanes are therefore sought.